From our over a decade-long experience in experiments and clinical applications of human hair keratin (HHK) artificial tendon, a conclusion was drawn that HHK artificial tendon components and their degradation products could stimulate the proliferation and differentiation of tenoblasts in the neighboring tissues into tenocytes. With the regulation of this process by certain cytokines secreted by other cells, autotendons can be finally formed. We also found that after grafting for the tissue defects such as in the bone, nerve and muscle, "in situ construction" of the tissue/organ substitutes occurred, which inspired us to propose a wholly new theoretical system--in vivo tissue engineering, defined as in vivo reconstruction of the defected tissues or organs. The grafted absorbable scaffold biomaterial itself and its degradation products can activate the mitosis, proliferation, and differentiation of adult stem cells in the surrounding tissues, which organically interact with the material to form an organic complex under in vivo physiological conditions. Finally the matrix material is completely replaced by the complex, an almost identical structure with the normal tissue in terms of anatomy and histology. One of the advantages of in vivo tissue engineering is "in vivo construction" of the tissue/organ substitutes in anatomy, histology and function way of "in vivo cultivation" of the seed cells under in vivo microenvironment and in vivo precise regulation mechanism. It solves such problems as immune rejection against seed cells, variation and functional deterioration of the seed cells, complicated preservation procedure, transportation and high cost. The most prominent advantage of this technique is that it best meets the clinical need, not only in the sense of its powerful clinical potential, but also in its significant theoretical value.